Hi Hell,
I think, the combiner works perfectly.
I think the difference between the two cases is that; there is a phase shift added to the output of the coupler, (check its Manual) while in the second case the two sources have random phases, that it is why there are some fluctuations of the output power with time.
Note that, the combiner by default add fields (see it Manual) unless the “Force power conservation” unchecked.
if it is checked you will find the two cases gives a stable 1 mW output with time.
Hope this clarify the case,
thanks
Mohamed Abouseif